Multiple combustion sensing device with false alarm prevention

ABSTRACT

Combustion-detecting means characterized by the combination of a pair of combustion-sensing mechanisms each responsive to and adapted to emit an electric signal upon occurrence of the same incident of a fire, but each having respectively different spurious or false alarm sensitivities, and an alarm trigger device responsive only to the sum of the signals, whereby the trigger device will be energized upon occurrence of the given incident of a fire but not upon occurrence of a false alarm condition to which either mechanism is sensitive.

United States Patent 72 Inventors Wilbur L. Ogden; 3,038,106 6/1962 Cutsogeorge. 317/1425 Clarence Glenn Henderson, both of Aurora, 3,074,053 1/ 1963 McDonough et a1. 340/258 111. 3,074,054 1/1963 Pearson 340/258 [211 App]. No. 775,484 3,505,641 4/1970 Boskovich..... 340/27 [22] Filed Nov. 13, 1968 2,807,006 9/1957 Collins et a1. 340/213 [45] Patented Oct. 5, 1971 3,078,450 2/1963 Bressler 356/207 X [73] Assignee BBK Electronics, Inc. 3,191,068 6/1965 Robb,Jr. 307/1 17 X Skokie,1ll. 3,245,067 4/1966 Blevins 340/228 3,255,441 6/1966 Goodwin et al... 340/228 X 3,268,881 8/1966 Vasel 340/228 X [54] MULTIPLE COMBUSTION SENSING DEVICE 3 364 476 ,19 Kahn u 340,2 3 WITHFALSE Q P 3,458,774 7/1969 Garshelis 340 213 x 15 Claims, 3 Drawing Figs. FOREIGN PATENTS [52] US. Cl 340/228, 729 1 3 19 Canada 307/111340/231340/420 1,073,217 6/1967 Great Britam [51] 1nt.Cl ..G08b 17/00,

Gosh 7/10, (30gb 19/00 P7171187) Examiner-John W. Caldvvell 50 Field of Search 340/420, 45mm Pamdge 227, 228, 237, 213, 258, 409, 412; 328/1, 2, 3 Atlurney-Gary, Parker, Juettner, Pigott & Cullinan [56] References Cited ABSTRACT: Combustion-detecting means characterized by UNiTED STATES PATENTS the combination of a pair of combustion-sensing mechanisms 3,541,539 1 1970 Tfumble 340/227 each responsive to and adapted to emit an electric signal upon 2,278,920 4/ 1942 Evans..... 340/237 occurrence of the same incident of a fire, but each having 2,465,377 3/ 1949 Jaeger 315/156 respectively different spurious or false alarm sensitivities, and 2,553,420 5/1951 McFee 338/17 an alann trigger device responsive only to the sum of the 2,702,898 2/1955 Meili 340/237 signals, whereby the trigger device will be energized upon oc- 2,759,174 4/1956 Brailsford.. 340/237 currence of the given incident of a fire but not upon occur 2,809,317 10/1957 Meili 313/226 rence of a false alarm condition to which either mechanism is 2,901,740 8/1959 Cutsogeorge 240/233 sensitive.

4 R X 123% 114% R7;

4 E; R2 Q4 PATENTEB um 5197i SHEET 2 BF 3 wk Q zz/zzawwfa m wwwewz m MULTIPLE COMBUSTION SENSING DEVICE WITH FALSE ALARM PREVENTION BACKGROUND OF THE INVENTION ln general, a fire is comprised of and passes sequentially through four stages; first, the incipient stage wherein only invisible products of combustion are given off and the fire is not humanly discernible because there is no smoke, flame, or sensible degree of heat; second, the smoke stage; third, the flame stage; and fourth, the heat stage generated by the flames. The first or incipient stage is generally of long duration, i.e., hours, days or weeks, and creates of itself practically no hazard to humans or property. Once the fire passes into the smoke stage, it progresses very rapidly and in minutes or even seconds will be characterized by smoke, flame and heat, each of which results in a high degree of hazard, causes property damage and perils human life.

Heretofore, mechanisms have been devised for detecting each stage of fire, i.e., the resistance bridge for detecting a fire in any of its stages, but primarily its incipient stage (see [1.8. Pat. No. 3,245,067 and copending applications, Ser. No. 709,415 and Ser. No. 751,086, now U.S. Pat. No. 3,548,205); the ionization type for detecting fire in its incipient stage (US. Pat. Nos. 2,465,377; 2,702,898; 2,759,l74; 3,078,450); the obscuration or photocell type for detecting smoke (U.S. Pat. No. 2,278,920); the photocell type for detecting flame or radiation (U.S. Pat. No. 2,553,420); and the thermal or thermister type for detecting heat (U.S. Pat. Nos. 2,901,740 and 3,038,106).

The smoke, flame and heat detectors are, of course, valuable; but by the time the incident of combustion that they are designed to detect occurs, the fire has progressed into its highhazard level where it is causing property damage and jeopardizing human life.

Consequently, major attention is focused upon the sensing mechanisms for detecting fire in its incipient stage, i.e., the resistance bridge type and the ionization type. Both types are sensitive to the invisible products of combustion produced by incipient fire conditions; the ionization type detecting the very small invisible particles, as well as smoke, and the resistance bridge detecting the very small particles and the water vapor that is given off as a product of combustion.

However, a principal problem encountered in incipient fire detection is false alarms caused by factors other than combustion. All types of fire detection devices are sensitive to atmospheric conditions other than combustion products; for instance, the ionization type is very sensitive to wind velocities, ozone, dust, and various chemical vapors, while the resistance bridge type is sensitive to intermittent fast humidity changes.

Therefore, while much has been accomplished, there has been no complete solution to the problem of early fire detection; or stated more accurately, early fire detection still suffers incidental problems particularly false alarms.

SUMMARY OF INVENTION This invention overcomes the problem of false alanns caused by factors other than combustion, and provides an improved early warning fire detector of far greater sensitivity and reliability than any heretofore available.

The invention is predicated on the concept of utilizing two or more principles of combustion detection in such fashion as to have each compensate for the limitations of the others, i.e., to combine principles wherein the noncombustion conditions to which each is sensitive are such that they rarely if ever occur simultaneously in nature.

Specifically, the invention combines at least two detection mechanisms, which are sensitive to different noncombustion conditions, with an alarm trigger device that is responsive to the sum of the alarm outputsof the two mechanisms but not to the alarm output of either one of themalone.

The consequence of the combination is that should a noncombustion condition occur-which would trigger one detection mechanism to alarm state, that same condition would not trigger the other and there would be no false alarm sounded by the alarm trigger device. By mating the two detection mechanisms in such a manner that each one compensates for or offsets the limitations of the other, i.e., the others sensitivities to noncombustion conditions, the likelihood of any false alarm occurring is reduced by many magnitudes, if not practically eliminated, inasmuch as at least two different and respectively critical noncombustion conditions would have to occur simultaneously to cause a false alarm.

The invention particularly provides a greatly improved early warning or incipient stage fire detector comprised of the combination in an AND gate circuit of a resistance bridge type of detection mechanism and an ionization type of detection mechanism.

Heretofore, the bridge type has suffered from undue sensitivities to fast changes in the water vapor content of the atmosphere while the ionization type has sufiered adverse consequences from chemical vapors, dust, ozone and highvelocity airflow. However, we have found that the ionization type is essentially insensitive to changes in water vapor content and that the resistance bridge type is essentially insensitive to airflow and, when properly compensated, to chemical vapors, ozone and dust. By combining these two mechanisms in an AND" gate system, where the sum of their alarm signals will trigger an alarm device but their individual signals will not, each offsets or compensates for the deficiencies of the other.

In the combination, a fast change in the water vapor content of the atmosphere will trigger the resistance bridge but not the ionization device; and particles of dust or a gust of air or ozone may trigger the ionization device but not the resistance bridge. However, when a fire starts, even in its incipient stage, both devices will respond to the same incident of the fire, i.e., the

products of combustion and the change in their respective electrical conductivities occasioned thereby. Specifically, the ionization device will quickly detect the small invisible particles of combustion, the resistance bridge will quickly detect these same particles and/or the sharp increase in the water vapor content of the air, and the sum of the two resulting signals will trigger the alarm.

Thus, the causes of false alarms in both of the units are largely if not entirely compensated for, whereby practically to eliminate false alarms, without diminution of each units capability for fast and early detection of a fire.

In fact, these units have previously been operated far below their detecting capabilities or sensitivities merely because of the false alann problem, i.e., the devices had to be rendered relatively insensitive for the purpose of minimizing the number of false alarms.

By virtue of the fact that we have essentially eliminated the false alarm problem, we can now operate the resistance bridge, and the ionization mechanism, and in combination therewith other detecting mechanisms as well, at much higher sensitivities than previously possible, essentially peak sensitivity, and thereby provide a detector more sensitive and rapid acting than ever before, and at the same time far more reliable.

Additional objects and advantages will become manifest as the description proceeds.

THE DRAWINGS FIG. I is a simplified schematic diagram exemplifying the concepts of the present invention;

FIG. 2 is a circuit diagram of a preferred embodiment of a detector for commercial and industrial use, which is adapted to be energized from a line and to transmit a signal back to a central alarm station; and

FIG. 3 is a circuit diagram of a preferred embodiment of a totally self-contained, battery operated fire detector and alarm particularly adapted for residential use.

DESCRIPTION in order to acquaint those skilled in the art with the manner of making and using our invention, we have shown and will now describe what we presently contemplate to be the best mode of carrying out our invention.

In essence, the invention comprises the combination in an AND" gate circuit of a pair of combustion-sensing mechanisms which are responsive to the same incident of a fire, or the change occurring in the same ambient condition upon combustion, but which are sensitive to respectively different spurious or false alarm noncombustion conditions of such character as would infrequently occur simultaneously, whereby to emit an alarm signal upon occurrence of the given incident of or ambient change caused by a fire but not upon occurrence of a noncombustion false alarm condition to which either mechanism may be sensitive. in this manner, each mechanism compensates for the shortcomings of the other and each may thus be made more sensitive than heretofore practicable. The preferred basic combination is a pair of mechanisms responsive to the incipient stage of fire. To this basic combination may be added additional types of sensing mechanisms, with the AND" gate set to respond to the sum of the signals of two, three or more of the mechanisms.

The ultimate is schematically illustrated in FIG. 1 wherein a resistance bridge mechanism 10, an ionization mechanism 20, a heat-detecting mechanism 30, a smoke-detecting mechanism 40 and a flame-detecting mechanism 50 are all connected in parallel across a suitable power supply.

The resistance mechanism is comprised of a variable impedance grid R exposed to the atmosphere and a compensating circuit CR connected in bridge circuit with the grid, and a primary trigger device PT, such as a MOSFET transistor, connected to the center tap of the bridge. This circuit or mechanism is described and claimed in copending application, Ser. No. 75l,086, now U.S. Pat. No. 3,548,205, which is an improvement over U.S. Pat. No. 3,245,067 and copending application, Ser. No. 709,415.

The ionization mechanism 20 is comprised essentially of an ionizing chamber I and a primary trigger device PT. Important advantages provided by this invention in the ionization mechanism are (l) elimination of the requirement for complete saturation of the ionizing chamber, with the consequent results of (2) reducing the mechanism to a single ionizing chamber, rather than two, and (3) reducing the amount of radioactive material required as an ionizing source. In fact, we are able to reduce the total amount of radioactive material to less than I microcurie, which is less than the amount usually found on a luminous watch dial. in the illustrated circuit, the ionizing chamber 1 is connected in a bridge circuit with a balancing device Bl, such as a fixed resistor, and the primary trigger is coupled to the center tap of the bridge.

The heat mechanism 30 is an essentially conventional rate of rise detector comprised of a thermister H freely exposed to atmosphere and a compensating thermister CH relatively shielded from atmosphere, the two being connected in a bridge circuit with a primary trigger device PT connected to the center tap of the bridge. As will subsequently appear, this heat-detecting mechanism may be made many magnitudes more sensitive than heretofore possible by virtue of its inclusion in the multiple gate system of this invention.

The smoke mechanism 40 may be an obscuration device comprising a light source L and a photocell S which is rendered conductive when smoke obscures the light beam, the photocell being appropriately connected to a primary trigger PT. Here again, it is to be noted that the photocell need not be compensated to act as a rate of change device, as has been true in the past. but can now be utilized as a positive measure of any predetermined degree of obscuration.

The flame-detecting mechanism 50 may suitably comprise a photocell F arranged to respond to a flickering flame, or a device sensitive to the radiation of flame, and is similarly coupled to a primary trigger PT without necessity for compensation, although compensation may sometimes be desirable.

The outputs of all of the primary trigger devices PT are connected to a gate or discriminating network DN so constituted as to reject or be nonresponsive to the signal emitted by any one of the primary triggers, but to respond to the sum of the signals emitted simultaneously be a preselected number of the primary triggers more than one. This network in turn is coupled to an alarm trigger AT adapted to be rendered conductive when the gate DN passes the preselected sum of signals, and is here shown as simply a switching device for connecting a bell, light or other alarm mechanism A across the power supply.

If the gate DN were set to pass the sum of any two output signals simultaneous activation of any two of the mechanisms 10, 20, 30, 40 and 50 would result in an alann. In the case of slowly occurring spontaneous combustion, the resistance bridge 10 and ionization mechanism 20 would quickly detect the situation and cause an alarm. If there were a dangerous buildup of heat starting to produce charring, the combination of the heat detector 30 and one or both of the mechanisms 10 and 20 would cause an alarm. If an incendiary device or an arsonist started a fire, the flame and smoke detectors 40 and 50, together with the mechanisms 10 and 20, would quickly cause an alarm to be sounded. Thus, complete fire detection would be provided.

Yet, the various noncombustion conditions to which each of the mechanisms is sensitive would not nonnally cause a false alarm. A flickering lamp bulb might trigger the mechanism 50, but absent a product of combustion sensible by one or more of the other mechanisms, there would be no alarm. A rapid increase in the relative humidity might trigger the mechanism 10, but absent a product of combustion sensible by one of the other mechanisms there would be no alarm. Likewise with dust, high-velocity airflow, chemical vapors, ozone, sudden changes in temperature, etc. The likelihood of two such noncombustion conditions occurring simultaneously in nature is extremely remote, and therefore the assembly is essentially free of he problem of false alarms.

Because the combination is thus free of false alarms, each of the mechanisms can be effectively utilized at essentially peak sensitivity. From a practical standpoint, sensitivity of prior fire detectors has been largely influenced by the problem of false alarms. All of the detecting mechanisms are capable of great detecting sensitivity, which by and large was not usable heretofore due to the innumerable false alarms encountered if the mechanisms were set to too high a sensitivity, and therefore the systems were necessarily operated far below their capabilities. By using two or more detecting mechanisms in an AND gate system, the problem is resolved with a consequent and substantial gain in sensitivity.

it will be manifest of course that the gate DN may be adjusted to cause an alarm only when three, four or all five of the mechanisms are triggered, and that any one or all of the detecting mechanisms 30, 40 and 50 may be omitted from the circuit depending upon the detecting capability desired.

In the preferred embodiments of this invention the detector preferably consists of just the resistance bridge 10 and the ionization mechanism 20 inasmuch as these units are sensitive to the incipient stage of fire, the change in the electrical conductivity of the air occurring upon combustion and the invisible products of combustion, i.e., water vapor, small particles and smoke which constitute products of practically all naturally occurring fires, as well as arsons, and are capable of detecting fires before life and property are imperiled by smoke, flame and heat. Referring now to FIG. 2, we have shown a preferred embodiment of a detector that is line powered and adapted to impose a fault on the line for alarm purposes, the detector being intended for use in multiple detector installations in commercial and industrial buildings. In this environment, it is customary to connect a plurality of detector heads toa common power supply leading from a central alarm station. There may be 20 or more detector heads on a single line connected to an alarm module applicable to a certain area of the building, the remaining areas of the building being covered by respective ones of a plurality of such modules, all under the supervision of a custodian who upon observing an alarm condition, given visually and/or audibly at the central station, may check to see what action is required, eg hand extinguishment, calling the fire department, evacuation of the area, etc. To facilitate this type of installation, and also to provide for ease of mounting and replacement of detector heads, a plug-in adapter 60 is provided for each head with the adapter wired into the line, as shown in the lower right-hand comer of FIG. 2. For cooperation with this adapter, the individual head is provided with a complementary plug 61 having pins 5 and 7 providing a power supply for the head. A pair of resistors R9 and R10 are provided to adjust the circuit to either 120 volt or 24 volt power supplies; both resistors being utilized at 120 volts and the resistor R10 being shunted by a jumper at 24 volts.

Connected in parallel across the power supply are a resistance bridge mechanism 10 and an ionization mechanism 20. The mechanism 10 comprises a comblike conductive grid mounted on a glass substrate as disclosed in U.S. Pat. No. 3,245,067 and application, Ser. No. 709,415, and constitutes a variable impedance R sensitive to the rapid increase in the conductivity of the air upon initiation of a tire, especially as occasioned by water vapor and the larger than usual but nevertheless invisible particles comprising other products of the combustion. This variable impedance is compensated in respect of naturally occurring ambient changes by a compensator circuit comprised of a MOSFET transistor Q2, a resistor R2 and a capacitor C1 which in essence act as a time delayed high resistance voltage regulator maintaining a constant voltage drop across the grid R, except when the resistance or conductivity of the grid changes a preselected amount within a preselected period of time. By appropriate valuation of the resistance and capacitance, the amount and the time can be varied to provide a carefully controlled compensation rate depending upon the type of fire to be detected and the prevailing atmospheric conditions, all as described in greater detail in application, Ser. No. 751,086, new U.S. Pat. No. 3,548,205.

Upon initiation of a fire, the conductivity of the grid changes at a rate faster than the circuit can compensate, and the bridge comprised of the grid and the circuit becomes sufficiently unbalanced to effect triggering of the primary trigger device Q1. The device Q1 preferably comprises a MOSFET having its source connected to the line through sensitivity adjustment means comprised of a diode chain indicated at D; its gate connected to the center tap of the grid bridge; and its output connected through resistance R3 to an alarm trigger device 05. Preferably, Q1 is biased so that it normally has no current flow, and the resistor R3 is selected to limit the current flow to a predetermined value, for example 130 microamperes, when Q1 is rendered conductive by operation of the detector grid R.

The ionization mechanism responds to the same incident of combustion as the mechanism 10, i.e., the larger than usual but nevertheless invisible particles resulting from combustion, and comprises a bridge circuit including an ionization chamber R5 containing a small amount of radioactive material, less than one microcurie, and a balancing resistor R4 for the chamber. The chamber R5 may be of any conventional form known in the art and the amount of radioactive material is such that the chamber is not saturated with ions. This causes the chamber to have very high impedance, and we have found this to be an asset. By operating the ion chamber in unsaturated condition, it is not sensitive to atmospheric pressure, and will work without compensation at all atmospheric pressures up to 25,000 feet without change in sensitivity. Also, because of the fewer ions created by the radioactive material, a smaller amount of combustion products are required to change the electron current flow. This allows then the use of a stable fixed resistor R4 for balance and provides highly sensitive detection with a very safe amount of radioactive material.

The ionization bridge is coupled to the gate of a primary trigger in the form preferably of a MOSFET Q3, the output of which, like that of Q1, is connected through resistor R7 to the alarm trigger Q5. Resistor R7, like resistor R3, serves to limit the current flow of 05 when triggered to a preselected value, for example microamperes. The source of O3 is coupled through a voltage regulating silicon transistor Q4 to sensitivity adjusting means, here shown in the form of a potentiometer R8. To provide normal energization, the transistor Q3 is biased to a relatively nominal current flow, for example 50 microamperes.

The alarm trigger device Q5 suitably comprises an SCR transistor having a gate current greater than the output of either of the MOSFETS Q1 and Q3 but less than the sum thereof, for example 200 microamperes in the example thus far given. Consequently, when the ionization chamber alone is conductive, the output of its trigger device 03 is limited by resistor R7 to 130 microamperes and Q5 will not be triggered. When the grid R alone is rendered sufficiently conductive to activate trigger Q1, the total current input to Q5 will be the sum of the l30-microampere trigger output of Q1 and the 50- microampere normal current flow of Q3, which still will not be sufficient to trigger Q5. However, upon initiation of a fire, each of the primary triggers Q1 and Q3 will have a current flow of 130 microamperes, providing a total current flow of 260 microamperes. As this occurs, the voltage drop across the resistor R6 becomes sufficient to turn on the SCR Q5, whereupon Q5 short circuits the power supply causing a significant line current draw capable of being sensed at a remote alarm panel for purposes of energizing an alarm device.

A lamp L1 is connected in circuit with the SCR to indicate by its illumination the particular detector head that has caused the alarm when a plurality of heads are connected to a common supply line. Preferably, a lead extends from the light circuit to terminal 4 on the plug 61 to provide for remote annunciation should this be desired, in which case appropriate circuitry is connected to terminal 4 of the adapter 60.

Also, conductors preferably extend from various points in the circuit to pins 1, 2, 3 and 6 of the plug 61 to facilitate ascertainment of voltage and current values for purposes of servicing the detector.

By virtue of the described circuitry, an extremely sensitive early warning fire detector is provided that has little if any susceptibility to false alarming under customarily encountered ambient conditions. The result is an improved and practically foolproof fire detector of vastly enhanced sensitivity and reliability; yet one that is completely compatible with conventional detector installations, wiring and alarm systems.

Referring now to FIG. 3, we have shown our dual gate-detecting system applied to a self-contained battery operated fire detector of the character described in our copending application, Ser. No. 709,415. This circuit is essentially the same as above described, except that it is battery powered and embodies its own alarm horn.

The circuit, the same as before, is comprised of a resistance bridge 10 including detector grid R and trigger device Q1; an ionization mechanism 20 including ion chamber R5 and trigger device Q3; and an alarm trigger device Q5 which in this case is connected in series circuit with a horn H and a battery B1.

The power for the system is supplied by a 10.7-volt mercury battery B1, which provides the small bias current to operate the detector mechanisms, and also provides the current necessary to drive the horn in the event of an alarm.

When the SCR Q5 triggers into the on condition, current flows through the horn H from battery B1, thereby sounding an alarm. Capacitor C3 serves to supply starting current for the horn thereby to relieve the load on the battery.

In this circuit, battery B1 is continually monitored by means of a supervisory battery B2 through a voltage-dividing potentiometer R1 1. When the current of battery B1 begins to fall off at the end of battery life, the bucking voltage of battery 81 is overcome by battery B2, whereupon capacitor C4 charges through resistor R12 to the point where a programmable unijunction transistor or PUT Q6 switches on, in turn triggering an SCR Q7. When Q7 is triggered on, it connects the battery B2 to the horn H thereby, with the aid of capacitor C5, to sound an alarm. SCR 07 can only remain switched on as long as gate current is supplied from the PUT Q6. Q6 remains on until capacitor C4 discharges through the PUT anode, whereupon Q6 shuts off, causing Q7 to shut ofi", and the horn to stop sounding. At this point, C4 again begins to charge resistor R11 from battery B2, and after an elapsed time of approximately minutes, the cycle is repeated. Thus, every 5 minutes the horn will blast from 1 to 2 seconds to signal the demise of battery B1 and the need to replace it.

As with the circuit described in connection with FIG. 2, energization of either of the detecting mechanisms by noncombustion conditions will not result in activation of the SCR OS; but upon occurrence of a fire both mechanisms are energized, Q5 is triggered on" and the horn is energized.

In this device, the circuitry is such that once Q5 is triggered it will remain triggered until reset. To accomplish resetting, a switch SW is provided which when closed will cause the triggered device Q5 to be bypassed, the load thereon relieved and the device thereby to be reset to nonconducting or off condition, whereupon the alarm is turned off until the rate of change of ambient conditions is again such (or remains such) that the device is again triggered.

By connecting the bypass to the switch contacts in such manner that the primary power source Bl becomes directly closed upon the horn H, the switch simultaneously provides a convenient means for manually checking the operability of the device and the condition of the battery B1.

In any of its embodiments, the dual or multiple gate system of the invention may be appropriately mounted within the housing shown in our application, Ser. No. 709,415.

Thus, our dual gate circuit has been shown to be equally applicable for both commercial and residential fire detection; to provide a highly sensitive and very reliable detecting system responsive to all stages of fire, but especially the incipient stage; and to essentially mitigate the problem of false alanns. In this manner the objects and advantages of the invention have been shown to be obtained in a convenient and economical manner.

For practically all commercial and residential fire-detecting systems, the AND gate combination of the resistance bridge and the ionization mechanism 20 will provide optimum results. However, any other detecting mechanism can be added to the circuit for special purposes. For example, the resistance bridge is sensitive to every type of fire, but the ionization mechanism is responsive only to incomplete combustion.

If it is desired to detect fires where complete combustion takes place (a rarity), an additional detecting mechanism can be added so that an alarm will sound when any two of the three detecting mechanisms are responding to a fire. Such third detecting mechanism would preferably comprise a very sensitive thermal rate of rise detector, employing a very small-detecting thermister of low thermal mass. In view of the safeguards of our multiple gate system, the sensitivity of such thermal unit could be adjusted to a rate as low as 1 F. per min., which is far more sensitive than any thermal unit heretofore usable. The thermal rate of rise mechanism would be in the form of an electric bridge, with the detecting thermister exposed to the atmosphere and having a low thermal time constant, while the second or reference thermister would have a much longer thermal time constant by being of larger physical mass, or being fastened to a part having a large thermal mass.

Thus, while we have shown and described what we regard to be the preferred embodiments of our invention, it is to be appreciated that various changes, rearrangements and modifications may be made therein without departing from the scope of the invention, as defined by the appended claims.

We claim:

I. A combustion detector comprising, in combination, a pair of combustion sensing means each responsive to the change in a given ambient condition occurring upon combustion and each sensitive. respectively that would infrequently occur simultaneously, said sensing means being connected in circuit with one another and each adapted to emit a sensible electric signal upon the occurrence of combustion, and trigger means connected in circuit with said sensing means nonresponsive to the individual signals of said sensing means but responsive to the sum of the signals of said sensing means for emitting an alarm signal.

2. A detector as set forth in claim I, wherein one of said sensing means comprises a resistance bridge sensitive to the change in the electrical conductivity of the air upon occurrence of combustion.

3. A detector as set forth in claim 2, wherein the second sensing means comprises an ionization type of combustion detector.

4. A detector as set forth in claim 3, including a third sensing means comprising a thermal type of combustion detector.

5. A detector as set forth in claim 1, wherein said sensing means are responsive to the incipient stage of fire.

6. A combustion detector comprising, in combination, a pair of combustion sensing means each responsive to invisible products of combustion and each sensitive respectively to different noncombustion or spurious alarm conditions that infrequently occur simultaneously, said sensing means being connected in circuit with one another and each adapted to emit a sensible electric signal upon the occurrence of combustion, gate means connected in circuit with said sensing means for rejecting the individual signals of said sensing means and for passing the sum of the signals of said sensing means, and signalling means energized by the said sum of signals passed by said gate means.

7. A detector as set forth in claim 6, wherein one of said sensing means is a resistant bridge sensitive to the change in the electrical conductivity of the air and comprises a sensor exposed to the air and an electronic circuit for automatically compensating said. sensor in respect of changes other than those of a predetermined magnitude occurring within a preselected time interval.

8. A detector as set forth in claim 6, wherein one of said sensing means is an ionization chamber operated in an unsaturated condition and utilizing a radioactive ionizing source of less than 1 microcurie.

9. A detector as set forth in claim 6, wherein one of said sensing means is a resistance bridge sensitive to the change in the electrical conductivity of the air and comprises a sensor exposed to the air and an electronic circuit for automatically compensating said sensor in respect of changes other than those of predetermined magnitude occurring with a preselected time interval, and the other of said sensing means is an ionization chamber operated in an unsaturated condition and utilizing a radioactive ionizing source of less than I microcurie.

10. A detector comprising, in combination, a pair of different sensing mechanisms each responsive to the change in electrical conductivity of air upon occurrence of combustion and each sensitive respectively to different noncombustion or spurious alarm conditions, said mechanisms being connected in circuit with one another and each subject to change in its electrical conductivity upon occurrence of respective conditions and adapted to emit a sensible electric signal upon occurrence of a given magnitude of change in its conductivity, gate means connected in circuit with said sensing mechanisms for rejecting the individual electric signals of said sensing mechanisms and for passing the sum of the electric signals of said sensing mechanisms, and signalling means energized by the said sum of the electrical signals passed by said gate means.

1 l. A combustion detector comprising a pair of combustion sensing mechanisms connected in parallel circuit, each responsive to the change in a given ambient condition occurring upon combustion and each sensitive respectively to different noncombustion spurious alarm conditions such that would infrequently occur simultaneously, one of said mechanisms comprising a resistance bridge comprised of a sensor and a compensator and a primary trigger device connected to the center tap of the bridge, another of said mechanisms comprising an ionization bridge comprised of an ionization chamber and a balance and a secondary primary trigger device connected to the center tap of the bridge, and an alarm trigger device having a gate connected to the outputs of said primary trigger devices, each of said primary trigger devices having a normal output and a trigger output, said alarm trigger device having a gate input greater than the sum of the nonnal output of one primary trigger device and the trigger output of the other primary trigger device but less than the sum of the trigger outputs of the two primary trigger devices.

12. A detector as set forth in claim 1 1, including an adjustable input for each of said primary trigger devices.

13. A detector as set forth in claim 11, said alarm trigger device having its output short circuited across the power source.

14. A detector as set forth in claim 11, said alarm trigger device having its output connected in series circuit with an alarming device across the power source.

15. A detector as set forth in claim 11, said ionization chamber being operated in an unsaturated condition and having a radioactive ionizing source of less than 1 microcurie.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,611,335 Dated October 5, 1971 Inventor(s) Wilbur L. Qgden 8c Clarence Glenn Henderson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, line 75, between "respectively" and "that" insert to different spurious alarm conditions, wherein the combustion sensing means are each responsive to different non-combustion changes in an ambient condition such Column 8, line 49, "with" should read --within--.

Signed and sealed this 21st day of March 1972.

(SEAL) Attest:

EDWARD 'M.FLETCHER, JR. ROBERT GOTTSCHALK .Attesting Officer Commissioner of Patents 

1. A combustion detector comprising, in combination, a pair of combustion sensing means each responsive to the change in a given ambient condition occurring upon combustion and each sensitive respectively that would infrequently occur simultaneously, said sensing means being connected in circuit with one another and each adapted to emit a sensible electric signal upon the occurrence of combustion, and trigger means connected in circuit with said sensing means nonresponsive to the individual signals of said sensing means but responsive to the sum of the signals of said sensing means for emitting an alarM signal.
 2. A detector as set forth in claim 1, wherein one of said sensing means comprises a resistance bridge sensitive to the change in the electrical conductivity of the air upon occurrence of combustion.
 3. A detector as set forth in claim 2, wherein the second sensing means comprises an ionization type of combustion detector.
 4. A detector as set forth in claim 3, including a third sensing means comprising a thermal type of combustion detector.
 5. A detector as set forth in claim 1, wherein said sensing means are responsive to the incipient stage of fire.
 6. A combustion detector comprising, in combination, a pair of combustion sensing means each responsive to invisible products of combustion and each sensitive respectively to different noncombustion or spurious alarm conditions that infrequently occur simultaneously, said sensing means being connected in circuit with one another and each adapted to emit a sensible electric signal upon the occurrence of combustion, gate means connected in circuit with said sensing means for rejecting the individual signals of said sensing means and for passing the sum of the signals of said sensing means, and signalling means energized by the said sum of signals passed by said gate means.
 7. A detector as set forth in claim 6, wherein one of said sensing means is a resistant bridge sensitive to the change in the electrical conductivity of the air and comprises a sensor exposed to the air and an electronic circuit for automatically compensating said sensor in respect of changes other than those of a predetermined magnitude occurring within a preselected time interval.
 8. A detector as set forth in claim 6, wherein one of said sensing means is an ionization chamber operated in an unsaturated condition and utilizing a radioactive ionizing source of less than 1 microcurie.
 9. A detector as set forth in claim 6, wherein one of said sensing means is a resistance bridge sensitive to the change in the electrical conductivity of the air and comprises a sensor exposed to the air and an electronic circuit for automatically compensating said sensor in respect of changes other than those of predetermined magnitude occurring with a preselected time interval, and the other of said sensing means is an ionization chamber operated in an unsaturated condition and utilizing a radioactive ionizing source of less than 1 microcurie.
 10. A detector comprising, in combination, a pair of different sensing mechanisms each responsive to the change in electrical conductivity of air upon occurrence of combustion and each sensitive respectively to different noncombustion or spurious alarm conditions, said mechanisms being connected in circuit with one another and each subject to change in its electrical conductivity upon occurrence of respective conditions and adapted to emit a sensible electric signal upon occurrence of a given magnitude of change in its conductivity, gate means connected in circuit with said sensing mechanisms for rejecting the individual electric signals of said sensing mechanisms and for passing the sum of the electric signals of said sensing mechanisms, and signalling means energized by the said sum of the electrical signals passed by said gate means.
 11. A combustion detector comprising a pair of combustion sensing mechanisms connected in parallel circuit, each responsive to the change in a given ambient condition occurring upon combustion and each sensitive respectively to different noncombustion spurious alarm conditions such that would infrequently occur simultaneously, one of said mechanisms comprising a resistance bridge comprised of a sensor and a compensator and a primary trigger device connected to the center tap of the bridge, another of said mechanisms comprising an ionization bridge comprised of an ionization chamber and a balance and a secondary primary trigger device connected to the center tap of the bridge, and an alarm trigger device having a gate connected to the outputs of saiD primary trigger devices, each of said primary trigger devices having a normal output and a trigger output, said alarm trigger device having a gate input greater than the sum of the normal output of one primary trigger device and the trigger output of the other primary trigger device but less than the sum of the trigger outputs of the two primary trigger devices.
 12. A detector as set forth in claim 11, including an adjustable input for each of said primary trigger devices.
 13. A detector as set forth in claim 11, said alarm trigger device having its output short circuited across the power source.
 14. A detector as set forth in claim 11, said alarm trigger device having its output connected in series circuit with an alarming device across the power source.
 15. A detector as set forth in claim 11, said ionization chamber being operated in an unsaturated condition and having a radioactive ionizing source of less than 1 microcurie. 